1. Field of the Invention
The present invention relates to an ink composition for inkjet recording, an inkjet recording method, and an inkjet recorded material.
2. Description of the Background Art
Inkjet printers are widely used due to their advantages such as low acoustic noises and low running costs and printers capable of printing color images on plain paper are introduced into the market actively.
However, it is extremely difficult to satisfy all the properties required of images produced using such printers, such as good color reproducibility, abrasion resistance, durability, light resistance, drying property, feathering, color bleeding, duplex printing, and ink discharging stability. As a result, ink is selected based on the particular application.
In general, the ink used inkjet recording is mainly composed of water, with a coloring agent and a wetting agent such as glycerin to prevent clogging. As the coloring agent, dyes are widely used for their coloring and stability. However, the light resistance and water resistance of images produced by using such dye-based ink are inferior.
The water resistance can be improved in some degree by using specialized recording media having an ink absorbing layer, but is not satisfactory at all when it comes to plain paper.
To compensate for such defects, ink using a pigment has begun to be widely used as a coloring agent in recent years. Although the pigment ink is successful and superior to the dye ink with regard to light resistance, water resistance, etc., the coloring is degraded by coherence of light having difference wavelengths and phases produced by multiple reflection of light inside the pigment. Therefore, the pigment ink is inferior to the dye ink in general with regard to the coloring.
In particular, the pigment ink used as ink for inkjet recording causes problems of deterioration of coloring (saturation) on plain paper and gloss on specialized paper.
In an attempt to compensate for such degradation of coloring of the pigment ink, pigment particulates that are coated with resin are used. According to this method, the fixing property and the gas resistance property of the ink are further improved by the resin, and in addition dispersion is greatly stabilized. However, currently, pigment ink is not still on a par with dye ink with regard to gloss.
For example, Japanese Patent No. 3301295 (JP-3301295-B) describes a method of manufacturing particles made of quinacridone, benzimidazolone, and azo pigments, which have a particle diameter of from 10 nm to 70 nm with an aspect ratio of from 1:1 to 1:2.
The aspect ratio regulates the ratio of the minor axis to the major axis. However, the aspect ratio is the same irrespective of the particle form in some cases. For example, it is 1:1 when the particle is a square shape or a star shape. This is fundamentally different from the inkjet ink in which ultra-fine organic particles having a sphere form sterically and a projection cross section of a circle are dispersed as they are, to be able to produce images with excellent color saturation and gloss while having excellent durability and discharging stability.
Japanese Patent Application Publication No. 2009-108199 (JP-2009-108199-A) describes ink having hydroinsoluble coloring material particles with a crystalline structure, a particle diameter of from 5 nm to 40 nm, and a simple dispersion degree of 1.5 or less.
Although this is successful in some degree, the coloring material particles are required to be spherical in shape to improve gloss and color saturation.
Therefore, ink having hydroinsoluble coloring material particles with a crystalline structure, a particle diameter of from 5 nm to 40 nm, and a simple dispersion degree of 1.5 or less is clearly different from ink having organic pigment particles formed to be spherical.